These experiments were designed to determine the susceptibility of fetal tissues to natural and synthetic estrogenic hormones as possible cancer-promoting or cancer-initiating substances. The effects of prenatal exposure to diethylstilbestrol (DES) and natural estrogens on the mammary glands and reproductive tracts of neonatal and adult rats will be examined. The incidence, growth, estrogen binding capacity (EBC) and morphology of mammary tumors arising in prenatally exposed glands, either spontaneously or after treatment with 7, 12-dimethylbenz(a)anthracene (DMBA), will be studied. Data from these experiments will provide insight into the sensitivity of embryonic tissues to hormones, and will define the periods during fetal life when tissues may be influenced so that the risk for developing cancer as an adult is altered.